Add publish_date and expiry_date support

devel/tests/helpers/is_visible.phpt

@@ -0,0 +1,21 @@
+--TEST--
+isVisible: returns true when metadata is null or date fields absent
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/context.php';
+require '/var/www/app/helpers.php';
+
+// Null metadata = always visible
+echo isVisible(null) ? "true\n" : "false\n";
+
+// Empty array = always visible
+echo isVisible([]) ? "true\n" : "false\n";
+
+// Metadata without date fields = always visible
+echo isVisible(['title' => 'Hello']) ? "true\n" : "false\n";
+?>
+--EXPECT--
+true
+true
+true